1. Field of the Invention
This invention relates to cooling of hot metal strip and more particularly to an improved method of and apparatus for cooling hot metal strip on the runout table of a hot strip finishing mill.
2. Description of the Prior Art
Metal strip produced on a hot strip finishing line is conventionally coiled by coilers spaced a substantial distance from the final stand of the finishing line. A runout table in the form of a driven roller conveyor supports the strip as it moves at high speeds from the stand to the coiler. The temperature of the strip issuing from the final roll stand however is too high for coiling and it is therefore necessary to cool the strip in its path along the runout table to avoid undesired metallurgical changes in the coil.
Strip issuing from a finishing mill is travelling at high rates and therefore rapid cooling on the runout table is required to avoid the necessity of an abnormally or unacceptably long runout table. Cooling is usually accomplished at a plurality of water spray stations or banks located along the runout table, with cooling water being applied both to the top and bottom surfaces of the strip. The bottom surface is cooled by spray bars positioned closely adjacent the strip, with the cooling water serving also to cool the metal surface of the conveying rollers. However, cooling water must be applied to the top of the strip from water delivery means spaced a substantial distance above the runout table in order to provide access to strip on the table and to avoid damage to the cooling system by cobbles which occasionally occur on the runout table as explained, for example, in U.S. Pat. No. 3,479,853.
In a typical hot strip finishing line, each cooling station includes a plurality of spray bars extending transversely of the strip above and below the runout table, with cooling water being directed onto the surface of the hot strip from a plurality of nozzles spaced along the length of each spray bar. The respective banks may consist of 10 to 12 individual spray bars above and below the strip with each spray bar being connected to a manifold supplied with water from a pressurized source, usually a separate pump for each bank, and valving is provided to control the flow of water to the individual banks. Preferably at least selected ones of the banks of spray bars also include valving to enable half sections of the bank to be selectively turned on and off to enable better control of strip cooling. In one hot strip finishing line operated by the assignee of the present invention, ten banks of twelve spray bars are provided above the strip with each bank receiving up to 3000 g.p.m. for a total water capacity of 30,000 g.p.m. applied to the top of the strip if all banks are operated simultaneously. This, of course, is in addition to the cooling water supplied to the bottom surface of the hot strip. The cooling water is recirculated and is therefore relatively hot and can be contaminated. Cooling systems having substantially greater water capacity are known and may be required particularly for heavier gauge strip or for strip leaving the final roll stand at higher temperatures or higher speeds.
Using the prior art system just described, it is generally possible to control the temperature only within the range of about 100.degree. F., or to about 50.degree. above or below the optimum or target temperature. In such a high volume water supply system, the valving necessarily requires substantial time to open or close during which time the flow rate varies, and upon closing there is a period of drainage or siphoning of the system during which some cooling water continues to flow from a closed bank. In a typical eight-hour shift, the valves might be actuated 250 or more times so that the valves, actuators and controls present a continuous maintenance problem.
One of the problems encountered in cooling hot strip is the difficulty in penetrating the layer of rapidly expanding steam which develops adjacent the hot top surface. Substantial water pressure is required, although excessive pressure may result in the cooling water being broken up into small droplets having insufficient inertia or momentum to penetrate the steam layer. A solution to this problem which is widely used in the industry, is the so-called laminar flow nozzle disclosed in U.S. Pat. Nos. 3,025,865 and 3,294,107 and which discharges water in a medium pressure coherent stream. By utilizing a plurality of such nozzles in closely spaced relation on each spray bar, the parallel streams of water penetrate the steam layer, effectively applying coolant across the complete width of the strip as it passes beneath each spray bar. Such laminar flow nozzles of course require relatively close control of water pressure and may be ineffective or only partially effective during periods of opening and closing of conventional header valves.
The use of quick-acting valves in each spray bar has also been considered as a means of avoiding the time delays inherent in systems which supply water either to a full or half bank of spray bars under control of a single cutoff valve. However, such systems introduce other problems including water surge in open lines when a valve controlling all or part of a bank is activated suddenly and water hammer in the headers and supply lines when a full bank is suddenly shut off. Also, the large number of fast acting valves in a system using contaminated cooling water, and the corresponding large number of controls and actuators required, present serious maintenance problems.
In an effort to overcome these problems, at least one attempt has been made to control the cooling rate while providing a continuous flow of water from the spray bars of a predetermined number of headers and by selectively deflecting the water from selected headers by use of a system of movable troughs supported above the strip. Power means move the troughs between a first position spaced from the spray bars to permit the water to descend onto the hot strip and a second position beneath the spray bars to catch the water and lead it over the side of the strip. Such a system does not provide a satisfactory solution, however, for various reasons. For example, relatively large troughs were required since water flow from a single spray bar could be up to 300 gallons per minute or more. The water discharged into the trough from relatively high pressure nozzles creates substantial turbulence in the trough and movement of the trough results in substantial spilling or splash-over from the trough itself. Further, since movement of the troughs necessarily has to be relatively slow, excessive spray is created during movement of the edge of the trough through the water streams. Such system also requires elaborate and heavy equipment to support and move the heavy troughs and due to the relatively large size and weight of the overall system, it is inherently slow acting. The relatively large troughs and their support system also presents substantial interference in the space above the runout table. Obvious difficulties with such a system may account for it apparently never having been widely accepted commercially.
A primary object of the present invention is, therefore, to provide an improved method and apparatus useful in the cooling of hot strip on a runout table.
Another object is to provide such a method and apparatus which enables a more positive and accurate temperature control and which avoids the foregoing and other problems encountered in the prior art cooling systems.